National Institute Of Materials Physics - Romania

In this work, we present results of the synthesis and characterization of iron and iron oxide nanoparticles aggregated in filamentary, spider-web-like structures. The particles were produced in a flow reactor by CO2 laser pyrolysis of gaseous mixtures of iron pentacarbonyl and ethylene. Low- and high-resolution electron microscopy reveals chain-like structures of particles, most of them being composed of an a-iron core and an iron oxide shell, identified as magnetite and, to a lesser extent, hematite. These results are in good agreement with a Mossbauer analysis carried out for the same samples. The role of the reaction temperature on the synthesis of filamentary iron nanostructures by infrared laser pyrolysis of Fe(CO)(5)/C2H4 mixtures is discussed.

Intermetallics of the type NdCo9-xFexSi2Cy (x = 3; 5; 6; 7 and y = 0; 1) and Nd2Co15-xFexSi2Cy (x = 7; 9; 12 and y = 0; 1) were prepared by are melting in order to study the competing effects of Fe and Co substitutions on the magnetic hyperfine fields as well as the effects of interstitial carbon additions on the symmetry and local magnetic properties in these systems. The investigations were performed by X-ray diffraction, Mossbauer spectroscopy and hysteresis loop measurements. Both site and average hyperfine magnetic field values were analyzed in order to discriminate between the Fe and Co contributions to the local atomic environments in these systems. While a higher Fe content as well as a higher Co content can both lead to an increase in the site magnetic fields, it was found that the model based on hyperfine field distributions provides a better description of the trends observed in hyperfine magnetic fields in these compounds. (C) 2001 Elsevier Science Ltd. All rights reserved.

Spray frozen, freeze dried nitrate powder with composition Bi:Pb:Sr:Ca:Cu 1.7:0.3:2.0:2.5:3.5 has been thermally decomposed (up to 870 degreesC). In situ neutron diffraction investigations during heating and cooling of the powder have been performed at DN2 instrument from the IBR2 reactor at JINR, Dubna, Russia. Phase formation and reformation and domains of stability for different phases were observed and discussed.

Dendritic structures were grown inside quartz crystals by electrodiffusion of sodium ions from a NaCl layer at the anode. An experimental arrangement using a point cathode and a graphite plate anode was employed. X-ray diffraction experiments were carried out, the structures chemical composition was found to be a mixture of sodium silicates. The chemical composition evolution in time was observed. There were made VIS and UV optical absorption spectroscopy measurements of these structures, a colloidal band being found. Thermoluminescence glow curves were measured. Based on the experimental results and observations a growth model is proposed.

An experimental study was made of the mechanical and electromechanical nonlinearities of a PZT trimorph type actuator. The actuator consisted of two piezoceramic poled plates, sandwiched on a thin metallic sheet, forming the usual trimorph structure, with one end fixed and the other one free. The experiments were conducted in two steps. In the first step a force was applied to the free end of the trimorph and its displacement was measured as a function of the applied force either for increasing and decreasing force. The nonlinearities in this case must be pure mechanical and they give information about the structural arrangement of the piezoceramic elements, glue and metallic sheet. For the second step the actuator was electrically activated with increasing and decreasing DC voltages and the displacement of the free end was determined as a function of applied DC voltage. The nonlinearities in this case are of electromechanical nature and they were the result of the combined effect of intrinsic material nonlinearities and structural arrangement. The C, experimental results showed that pure mechanical nonlinearities are practically inexistent even for forces up to 0.7 N. The displacement of the free end follows the theoretical predictions. When the actuator was electrically activated the displacement of the free end was entirely hysteretic and the electromechanical nonlinearities were different from the theoretical predictions. The results were discussed in terms of the displacement of ferroelastic-ferroelectric domain walls.

The structure of Fe/GaN/Fe ferromagnetic electrodes is studied by high resolution transmission electron microscopy. The layers grow epitaxially on the GaAs substrate with the top Fe layer 90degrees rotated compared to the bottom one. The interfaces are quite rough. There is an indication of the possible occurrence of Fe3GaAs formation on the GaAs interface.

This paper presents some of the more relevant results concerning the mass-loading influence on quartz resonators and monolithic filter characteristics, and recent experimental results about the mass-loading effect on electrical parameters of langasite resonators. Based on the Ballato's transmission-line analogs of the trapped-energy resonators vibrating in thickness-shear mode, the mass-loading effect on resonator characteristics was studied. The effective mass-loading, motional inductance and quality factor of quartz and langasite resonators, as a function of harmonic order for various electrode parameters, were computed. In the frame of the Tiersten's analysis of monolithic structure, the theoretical calculations of the coupling coefficient of two-pole quartz and langasite monolithic filters as a function of mass-loading were performed. The analysis of these results shows that the mass-loading influence on characteristics of langasite resonators and monolithic filters is lower than on those of the quartz devices.

The W-band (95 GHz) ODMR spectra of AgCl nanocrystals (NCs) embedded in a KCl single crystal lattice have been measured for various orientations of the magnetic field in a 100} crystal plane, Only self-trapped exciton (STE) signals are observed while no ODMR is found from distant electron-hole pairs. This confirms the conclusion of a previous ODMR study in the Q-band (at 35 GHz) concerning the spatial confinement effect and impurity exclusion on exciton recombination in NCs of AgCl. Through increased resolution, more accurate g- and zero-field splitting values of the triplet state were obtained in good agreement with those of the STE in bulk AgCl.

In this paper the results of electrical measurements of the mass-loading influence on AT-cut quartz resonator parameters are compared with those obtained by X-ray topography analysis of the same resonators. Based on the Ballato's transmission-line analogs of the trapped-energy resonators vibrating in thickness-shear mode, the mass-loading effect on resonator characteristics was studied and the effective mass-loading, motional inductance and quality factor of quartz resonators were computed. X-ray topography measurements were performed by transmission diffraction using the synchrotron radiation. Sawyer plan-parallel polished AT quartz resonators with 14mm diameter, 5MHz frequency, Au electrodes with various structures, diameters and thickness were used in experiments. The results of the X-ray topography investigations are in agreement with the previous ones obtained by electrical measurements [3,5,6,9].

This work describes the study of PbSe1-xTex film electrodeposition on glassy carbon and ITO electrodes in acid solutions (0.1 M HNO3), using cyclic voltammetry and rotating-disk techniques. Electrochemical impedance spectroscopy study of PbSe1-xTex film in 0.1M HNO3 were also presented.